In normal and iron-deficient adults, 80 to 90% of absorbed iron is promptly incorporated into erythrocytes, and erythrocyte incorporation of an administered iron isotope can therefore be used as an estimate of iron absorption rather than the more difficult and expensive metabolic balance method. Much of our current knowledge about iron absorption by adults has been obtained through studies of erythrocyte incorporation of radioisotopes of iron. The objection to using radioisotopes for similar studies of infants can now be circumvented by use of stable isotopes of iron, but satisfactory interpretation of the data obtained requires information about the extent to which erythrocyte incorporation of iron by infants is predictive of iron absorption. Using the stable isotope of iron, 58Fe, we propose concurrent determination of 58Fe balance and erythrocyte incorporation of 58Fe to test the following hypotheses: (1) Contrary to a widely held supposition, normal term infants absorb iron as efficiently during the first 4 mo of life as later in infancy. (2) Substantially less than 80% of absorbed iron is promptly (i.e., within 14 days) incorporated into erythrocytes by infants, at least by infants less than 4 mo of age. (3) The level of iron in the diet is a major factor determining the extent of prompt (i.e., within 14 d) incorporation of an orally administered iron isotope into erythrocytes. To accomplish Specific Aim 1, 58 Fe balance studies will be carried out in a fully longitudinal design with each of 12 infants studied at each of 4 ages (1, 2, 4 and 8 mo). Fecal collections will be made in the home until the 58Fe content of feces returns to baseline values. To accomplish Specific Aim 2, we shall determine the prompt (i.e., within 14 d) erythrocyte incorporation of the 58Fe label in the same infants studied in Specific Aim 1. To accomplish Specific Aim 3, we shall determine the prompt erythrocyte incorporation of iron in 2 groups of 12 infants each. After administration of 58Fe, one group will be fed a formula with iron content of 1 mg/L and the other will be fed a formula with iron content of 12 mg/L. 58Fe determinations in blood and feces will be made with the new Finnigan Sola ICP/MS instrument.